FIG. 6 shows a tube for a refrigerant condenser described in U.S. Pat. No. 6,595,273 corresponding to JP-A-2004-3787. Two plate portions are disposed opposite each other so that a flat tube 50 is configured. Each of the plate portions includes base plate portions 51 and multiple protruding portions 52, which protrude outward from the base plate portions 51.
The protruding portions 52 are disposed to be overlapped each other. Thereby, clearances are formed between the adjacent protruding portions 52. The clearances function as refrigerant passages (internal fluid passages) in which a refrigerant (an internal fluid) flows as shown by a dashed arrow “y”.
The protruding portions 52 include first protruding portions 52a and second protruding portions 52b. The protruding dimension of the second protruding portions 52b from the base plate portions 51 is smaller than the protruding dimension of the first protruding portions 52a from the base plate portions 51. The first protruding portions 52a are formed to extend meanderingly in a tube width direction, that is, in an up-down direction in the example of FIG. 6. The second protruding portions 52b and the base plate portions 51 are disposed alternately between the adjacent first protruding portions 52a. 
Therefore, air passages (external fluid passages), in which air flows in the tube width direction as shown by an arrow “z”, are formed by the second protruding portions 52b and the base plate portions 51.
Because the air at the periphery of the external surface of the tube 50 flows in the air passages meanderingly as shown by the arrow “z”, an air current is disturbed and generation of a thermal boundary layer at the periphery of the external surface of the tube 50 is suppressed. Thereby, thermal conductivity of the air is increased in the refrigerant condenser.
Hatching regions in FIG. 6 show brazing portions at which one plate portion is brazed with the other plate portion. That is, one plate portion is brazed with the other plate portion at the base plate portions 51 and end portions 53. Therefore, the base plate portions 51 can function as inner pillars for increasing pressure-resistance strength of the tube 50.
In the above-described tube, a brazing material will be filled in brazing between the opposite base plate portions 51 about at contact points of the base plate portions 51 of one plate portion and the base plate portions 51 of the other plate portion. Thereby, the brazing can be effectively performed.
When the base plate portions 51 of one plate portion and the corresponding base plate portions 51 of the other plate portion are not in contact with each other before the brazing due to the manufacturing error or the like, the brazing material is not filled between the opposite base plate portions 51 because of a lack of the contact points. Thereby, defective brazing may occur.
In the above-described tube, many separate base plate portions 51 are formed in dot shapes. Therefore, high manufacturing accuracy is required to connect all the opposite base plate portions 51 each other. It is difficult for the contact points to be provided at all the corresponding base plate portions 51. Thereby, brazing property between the two plate portions becomes worse.
The defective brazing also occurs in a tube, in which only one plate portion includes the first and second protruding portions 52a and 52b and the other plate portion has a flat shape.